It is known to acquire volumetric imaging data that is representative of an anatomical region of a patient or other subject. Volumetric imaging data may be acquired in advance of performing a medical procedure on that anatomical region. Volumetric imaging data that is acquired in advance of a procedure may be referred to as pre-operative volumetric data.
Pre-operative volumetric data may be obtained using any suitable three-dimensional imaging modality, for example computed tomography (CT), magnetic resonance (MR) or cone-beam CT (CBCT).
In the discussion below, a set of volumetric image data may also be referred to as an image volume, a three-dimensional (3D) image, or a 3D scan.
It is known to acquire live 2D imaging data while a medical procedure is being performed. The live 2D imaging data may comprise a stream of 2D images that are acquired in real time or near-real time.
Live 2D imaging that is performed during a medical procedure may also be referred to as intra-operative 2D imaging or real-time 2D imaging.
Intra-operative 2D images may be obtained using any suitable 2D imaging modality. For example, 2D fluoroscopy images may be obtained using an X-ray scanner, or 2D ultrasound images may be obtained using an ultrasound scanner.
Intra-operative 2D images such as fluoroscopy may be used for live guidance of medical devices towards (or away from) specific anatomical targets. A clinician may use the stream of live 2D images to determine a current position of a medical device within a patient's body, and to navigate the medical device through the patient's body in real time. For example, the medical device may be a needle or a catheter. A specific anatomical target may be any anatomical target that is visible in the 2D data, for example a tumor, bone, lung, or blood vessel.
Methods are known in which both a pre-operative 3D image and a stream of 2D intra-operative images are presented to a user, for example a clinician. The pre-operative 3D image may assist the clinician in guiding the medical device to (or away from) an anatomical target. The anatomical target may be any anatomical target that is visible in both the 2D and the 3D data, for example a tumor, bone, lung, or blood vessel.
The 3D data may provide information that is not available from the 2D data alone. The presence of a third dimension may provide the clinician with additional navigational information. In some circumstances, the 3D data may be of higher resolution than the 2D data. In some circumstances, the 3D data may be acquired using a different imaging modality from the imaging modality used to acquire the 2D data. The use of the different imaging modality may provide additional information to the clinician.
It may be considered that a frontier in image-guided intervention is the reliable alignment of pre-operative volumetric data with intra-operative 2D images.
To provide the user (for example, clinician) with useful information, a coordinate system of the volumetric data should be aligned with a coordinate system of the intra-operative 2D images.
Once the coordinate systems are aligned, images may be presented such that the position of anatomical features in an image rendered from the 3D data is the same as the position of those anatomical features in the 2D images. A fusion image may be obtained by fusing data from a 2D image and 3D image.
To align a 2D image and a 3D image, a 2D/3D registration process may be performed. 2D/3D registration may be time-consuming and/or computationally complex.
In some forms of 2D/3D registration, a Digitally Reconstructed Radiograph (DRR) is rendered from the 3D image. A DRR is a type of 2D projection of a 3D scan. The DRR is then registered to the 2D image.
Rendering a DRR may be time intensive. A single 2D/3D registration process may comprise multiple iterations of obtaining a DRR. The use of multiple DRR iterations may increase the time taken for the 2D/3D registration.
A DRR may have different image properties (for example, intensity and contrast) from a 2D image. Due to the different image properties, it may be the case that a multi-modality registration method (for example, mutual information) is used to register the DRR to the 2D image. In some circumstances, multi-modality registration methods may be more complex, more time-intensive and/or less accurate than methods for registering images of the same modality.
FIG. 1 shows an example of a fluoroscopy image of an anatomical region, which in the example of FIG. 1 is part of the spine. FIG. 2 shows a DRR that has been created from 3D data that is representative of the same anatomical region. The DRR is scaled to be of a similar scale and magnitude as the 2D data. However, it may be seen that the DRR of FIG. 2 and the fluoroscopy image of FIG. 1 do not look alike. In particular, the intensity values are very different.